Articles known as pneumatic or gas springs, which for convenience can be referred to as counterbalance links have become common in commercial articles, particularly in the automotive industry, but they are being used in many other fields wherever the need is present to provide a counterbalance force for closure units, such as lids, doors and cabinet fronts, and gas spring replacement for mechanical spring fittings has become prevalent. In some fields of use, particularly in the automotive field where pneumatic springs are used on each side of trunk lids and hatch backs, the springs counterbalance the weight of the lid or hatch back. The gas springs are retracted, i.e., the spring piston rod is moved into the spring cylinder, when the lids are closed. Then the gas spring units can extend, under gas pressure force acting on the effective piston shaft cross-section area, to move the lid to an open position. As is true of many gas springs, a control orifice by-pass in the gas spring, effectively slows the opening speed of the lid.
In the automotive field, particularly where pneumatic springs are used on hoods, trunk lids and hatch backs, two springs are used, primarily to provide balanced spaced apart forces to counterbalance the weight of the closure and prevent twisting or warping of the closure unit relative to its hinge axis. This invention while developed relative to automotive uses is not restricted to such field. It results in a compact installation using a single pneumatic spring connected through bell cranks and idler links to a hinged closure member. The elongate spring is mounted on and between bellcranks with the spring floating, between positions, in a direction transverse to the hinge axis of the closure. In conjunction with the counterbalance apparatus the pneumatic spring construction has been improved to provide a desired multi-output force feature as well as incorporating a unique method and structure to effectively decrease loss of operative gas pressure within the pneumatic spring resulting primarily from leakage of the gas directly through the shaft seal due to permeability of the material from which the seal is made. The latter aspect can be referred to as permeability compensation or pressure decay compensation.
Examples of transverse springs with bellcrank linkage are seen in the following U.S. Patents: U.S. Pat. Nos. 3,724,797 to H. Freitag et al for Resilient Seat; 4,416,094 to F. Bugener et al for Attic Window Assembly; and Re. 26,162 to A. K. Simons et al for Vehicle Seat Rebound Control. An example of a different dual output force concept in a pneumatic spring can be seen in applicant's U.S. Pat. Nos. 4,451,964 (also 4,451,978) where a floating piston is utilized to provide the dual output force. While not teaching the pressure decay compensation invention of this application, U.S. Pat. No. 4,408,751 to Daniel P. Dodson and George C. Ludwig for Multi-chamber Temperature Compensated Pneumatic Counterbalance shows a fixed partition or wall module providing a separate gas chamber in a pneumatic counterbalance cylinder.